Field
The present invention generally relates to the field of semiconductors. More particularly, the invention relates to semiconductor-on-insulator (SOI) devices.
Background Art
Transistors fabricated on a semiconductor-on-insulator (SOI) substrate (hereinafter referred to as “SOI transistors”), such as N-channel Metal Oxide Semiconductor (NMOS) and P-channel MOS (PMOS) SOI transistors, can be utilized in RF switches in electronic devices, such as cellular phones. For example, multiple SOI transistors can be coupled in series to provide an RF switch capable of handling the power levels required in a cellular phone. However, capacitive coupling between source/drain regions of the SOI transistor and an underlying bulk silicon substrate can adversely affect SOI transistor performance by, for example, providing an RF signal path to ground.
In one conventional approach, capacitive coupling in SOI substrates can be reduced by increasing the thickness of an underlying buried oxide layer, which is situated between the top silicon layer and the bulk silicon substrate. However, increasing the thickness of the buried oxide layer oxide beyond approximately 1.0 micron can increase strain in the SOI substrate, which can cause undesirably warping in the wafer. In another conventional approach, a high resistivity sapphire substrate can be used in place of the bulk silicon substrate. Although the sapphire substrate is effective in reducing capacitive coupling, the sapphire substrate significantly increases manufacturing cost.
Another conventional approach for reducing capacitive coupling in SOI substrates utilizes a high-resistivity bulk silicon substrate, such as a bulk silicon substrate having a resistivity of up to 1000 Ohms-cm. However, various effects, such as trapped charge in the buried oxide layer or at the interface between the buried oxide layer and the bulk substrate can induce a surface charge on the bulk substrate. As a result, a surface conducting layer can form on the bulk substrate, which can undesirably reduce the overall resistivity of the bulk substrate and, thereby increase capacitive coupling in the SOI substrate. The surface conducting layer can also provide an undesirable RF signal path between adjacent SOI transistors.